One class of opto-electrical devices is that using an organic material for light emission (an organic light emitting device or “OLED”) or as the active component of a photocell or photodetector (a “photovoltaic” device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer.
In an organic electroluminescent device, electrons and holes are injected into a layer of electroluminescent semiconducting material where they combine to generate excitons that undergo radiative decay. Holes are injected from the anode into the a highest occupied molecular orbital (HOMO) of the electroluminescent material; electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the electroluminescent material. An organic hole injecting material is commonly provided to assist injection of charge from the anode into the electroluminescent layer. In WO 90/13148 the organic light-emissive material is a conjugated polymer, namely poly (p-phenylenevinylene) (“PPV”). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). Light emitting polymers such as polyfluorenes and polyphenylenes are advantageous in that they are solution processable. In particular, solution processable light emitting polymers may be inkjet printed as described in EP 0880303 to produce high information content displays, in particular full colour displays.
An essential requirement for an effective OLED is efficient injection of holes and electrons into the electroluminescent layer of the OLED. Therefore, a focus in the OLED field is the development of hole and/or electron transporting materials to be used in combination with the electroluminescent material. An effective hole transporting material has a HOMO level that lies between the workfunction of the anode (or organic hole injecting material) and the HOMO level of the electroluminescent material.
Another focus in the OLED field is the development of full colour OLEDs, i.e. OLEDs comprising red, green and blue electroluminescent materials. A drawback of many blue organic electroluminescent materials is that their emission is a relatively pale blue, due to an insufficiently large HOMO-LUMO bandgap, when compared to the standard blue as defined by 1931 CIE co-ordinates.
WO 99/48160 discloses hole transporting co-polymers “TFB” and “PFB”:

These materials may be used as hole transporting materials and/or as blue electroluminescent materials. However, the above identified co-polymers have drawbacks. In particular, the HOMO levels of these materials are not ideally matched to the workfunction of the typical anode and/or organic hole injecting material, which negatively affects their hole transporting properties. Furthermore, the blue emission from these materials is a relatively pale blue.
There therefore exists a need for polymers having good hole transporting properties and a deeper blue colour. Phosphine-containing polymers are known. For example, EP 0339424 discloses polythioarylenes with phosphine repeat units. Repeat units within this polymer are not conjugated together, but are separated by a sulfur atom. These polymers are described for use in fields taking advantage of their inert, thermoplastic properties and as such the teaching of this document is not relevant to the field of the present invention of organic semiconductors.
J. Organomet. Chem. 653, 167-176, 2002 discloses a homopolymer comprising a diphenylalkylphosphine repeat unit.
WO 99/32537 discloses polymers comprising repeat units each of which consists essentially of units Ar3Y where Y can be N, P, etc. Copolymers of units Ar3Y with other repeat units are not mentioned. In the preferred embodiment of this application, Y represents nitrogen; there are no embodiments wherein Y is phosphorus or any indication of advantages of phosphines over the exemplified amines. Furthermore, there is no disclosure of how copolymers comprising such repeat units may be synthesised.